This is one of those subjects that even more controversial then castration and EPDs.
Years ago we used to go in and physically disconnect the placenta, then the deal became just giving them oxytocin, now the norm is to leave it alone and let it disconnect and come out/off by itself. We tie it up so that it doesn't drag the ground and the cow can't step on it and pull. Yes it gets nasty and stinks, but it will eventually disconnect and come out. It can cause a delay in breeding back, but I feel that just the problem that caused the retnetion in the first place is as likely to cause problems as the length of time it is retained.
Some vets still like to go in and manually disconnect it, some stll give the shot, some just leave it alone. The worst thing you can do is pull too hard and damage the uterus. The more rotten it is the less thechance of causing damage. If after 2 weeks it hasn't come out and the heifer cleaned, then is the time to do something. Keep an eye on her for signs of fever and let old ma nature do her thing.
dun